logistics (rostering, work-life balance, and internal communication) frustrated autonomy, and unfamiliarity with COVID-19 frustrated competence. Despite many need frustrating experiences, a strong connection with colleagues and patients were important sources of relatedness support (i.e. need satisfaction) that seemed to mitigate psychological distress. Conclusion The COVID-19 pandemic resulted in an increase of psychological distress among frontline workers. Both need frustration and need satisfaction explained unique variance of psychological distress, but seemed to originate from different sources. Challenging times require healthcare organisations to better support their professionals by tailored formal and informal support. We propose to address both indirect (e.g. organisation) and direct (e.g. colleagues) elements of the clinical and social environment in order to reduce need frustration and enhance need satisfaction. Citation: van der Goot WE, Duvivier RJ, Van Yperen NW, de Carvalho-Filho MA, Noot KE, Ikink R, et al. (2021) Psychological distress among frontline workers during the COVID-19 pandemic: A mixed-methods study. PLoS ONE 16(8): e0255510. https://doi.org/10.1371/journal.pone.0255510 Editor: Jianguo Wang, China University of Mining and Technology, CHINA Received: June 11, 2021; Accepted: July 18, 2021; Published: August 5, 2021 Copyright: © 2021 van der Goot et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: Due to local regulations and restrictions related to the confidentiality of participants, data are not publicly available. Requests for data availability can be addressed to the programme leader of LEARN (learn@umcg.nl). Each request will be evaluated on a case-by-case basis in accordance with the institutional policies of the University Medical Center Groningen, The Netherlands. Funding: The author(s) received no specific funding for this work. Competing interests: The authors have declared that no competing interests exist. Introduction In March 2020, the World Health Organisation declared the coronavirus disease 2019 (COVID-19) outbreak as a pandemic [1]. The COVID-19 pandemic has had an enormous impact on both healthcare systems and healthcare professionals [2]. Knowledge from previous virus outbreaks (e.g. SARS, MERS, and Ebola) indicated that healthcare professionals in contact with affected patients (so-called frontline workers) experienced more psychological distress and were more at risk for post-traumatic stress symptoms than healthcare professionals who had not been in contact with affected patients [3–6]. Based on previous research, there are strong indications that novel virus outbreaks may increase psychological distress among frontline workers during COVID-19. Emerging research on COVID-19 from several countries indicates that frontline workers report symptoms of depression, anxiety, and stress [7–9]. Reported risk factors included lack of perceived psychological preparedness and self-efficacy to help patients, social isolation, and fear of improper use or unavailability of personal protective equipment (PPE) and associated infection risks [10–12]. This is problematic, because psychological distress may lead to ill-being, which has detrimental effects on professionals, but also on healthcare organisations and patient safety.